Final answer:
In the third trimester of pregnancy, progesterone is primarily produced by the placenta.
Step-by-step explanation:
During the third trimester of pregnancy, progesterone is primarily produced by the placenta. Before pregnancy and in the early weeks, progesterone is produced by the corpus luteum in the ovaries. However, as pregnancy progresses, particularly after the first trimester, the placenta takes over the role of the primary endocrine organ, producing not only progesterone but also estrogen and hCG (human chorionic gonadotropin) to maintain pregnancy and support fetal development.
Progesterone is an endogenous steroid hormone that is commonly produced by the adrenal cortex as well as the gonads, which consist of the ovaries and the testes. Progesterone is also secreted by the ovarian corpus luteum during the first ten weeks of pregnancy, followed by the placenta in the later phase of pregnancy.